Pressure-induced symmetry changes in body-centred cubic zeolites

Previous work has shown a strong correlation between zeolite framework flexibility and the nature of structural symmetry and phase transitions. However, there is little experimental data regarding this relationship, in addition to how flexibility can be connected to the synthesis of these open-framework materials. This is of interest for the synthesis of novel zeolites, which require organic additives to permutate the resulting geometry and symmetry of the framework. Here, we have used high-pressure powder X-ray diffraction to study the three zeolites: Na-X, RHO and ZK-5, which can all be prepared using 18-crown-6 ether as an organic additive. We observe significant differences in how the occluded 18-crown-6 ether influences the framework flexibility—this being dependent on the geometry of the framework. We use these differences as an indicator to define the role of 18-crown-6 ether during zeolite crystallization. Furthermore, in conjunction with previous work, we predict that pressure-induced symmetry transitions are intrinsic to body-centred cubic zeolites. The high symmetry yields fewer degrees of freedom, meaning it is energetically favourable to lower the symmetry to facilitate further compression.

This a great work in zeolites world. I have a couple thoughts to share. First, do you think the discrepancy of compressibility betweeen ZK-5, RHO, and NaX is simply due to the larger beta-cage, that is 12-ring vs. 8-ring? Also, it's well accepted that the zeolite pores do possess certain flexibility, it would be nice to reference some simulation or other experimental data for comparison.

22-May-2019
Dear Dr Sartbaeva: Title: Pressure-induced symmetry changes in body-centred cubic zeolites Manuscript ID: RSOS-182158 Thank you for submitting the above manuscript to Royal Society Open Science. On behalf of the Editors and the Royal Society of Chemistry, I am pleased to inform you that your manuscript will be accepted for publication in Royal Society Open Science subject to minor revision in accordance with the referee suggestions. Please find the reviewers' comments at the end of this email.
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Once again, thank you for submitting your manuscript to Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry. I look forward to receiving your revision. If you have any questions at all, please do not hesitate to get in touch. ********************************************** RSC Associate Editor: Comments to the Author: (There are no comments.) RSC Subject Editor: Comments to the Author: (There are no comments.) ********************************************** Reviewer comments to Author: Reviewer: 1 Comments to the Author(s) This paper addresses a sparsely investigated, yet very interesting, area of research pertaining to the influence of pressure on the symmetry of zeolites prepared by using 18-crown-6 ether as an organic additive. Three zeolites, that is Na-X, RHO and ZK-5 have been studied, along with their empty analogs (zeolites for which the crown ether has been removed by thermal treatment). A systematic study of their crystal structure performed by high-pressure X-ray diffraction revealed that the synthesis of zeolites has a significant impact on their crystal structure and, consequently, on the flexibility of their networks. In particular, the authors suggest that the crown ether molecule acts as a geometric template in the synthesis of RHO and ZK-5 zeolites, behavior which is not displayed in the synthesis of the Na-X zeolite. Such a behavior has been tentatively ascribed to the body-centered structure of the zeolites whereby compression of the structural framework is allowed without influencing the rigidity of the networks. Based upon the experimental results, the authors postulate that such a behavior will be absent in the case of non-body-centered aluminosilicate zeolites. The manuscript is well-articulated, the discussion and conclusions being well supported by the experimental data. Perhaps it will be useful for the reader to know what is the experimental error in the determination of the unit cell parameters and cell volumes with the pressure. based upon the foregoing, I recommend the publication of this manuscript after the authors address the comments indicated above.

Reviewer: 2
Comments to the Author(s) Dear Authors, This a great work in zeolites world. I have a couple thoughts to share. First, do you think the discrepancy of compressibility betweeen ZK-5, RHO, and NaX is simply due to the larger beta-cage, that is 12-ring vs. 8-ring? Also, it's well accepted that the zeolite pores do possess certain flexibility, it would be nice to reference some simulation or other experimental data for comparison.

RSOS-182158.R1 (Revision)
Review form: Reviewer 2 Is the manuscript scientifically sound in its present form? Yes

Comments to the Author(s)
The revision appears good to me. Comments to the Author(s) The revision appears good to me.

Reviewer: 1
Comments to the Author(s) This paper addresses a sparsely investigated, yet very interesting, area of research pertaining to the influence of pressure on the symmetry of zeolites prepared by using 18-crown-6 ether as an organic additive. Three zeolites, that is Na-X, RHO and ZK-5 have been studied, along with their empty analogs (zeolites for which the crown ether has been removed by thermal treatment). A systematic study of their crystal structure performed by high-pressure X-ray diffraction revealed that the synthesis of zeolites has a significant impact on their crystal structure and, consequently, on the flexibility of their networks. In particular, the authors suggest that the crown ether molecule acts as a geometric template in the synthesis of RHO and ZK-5 zeolites, behavior which is not displayed in the synthesis of the Na-X zeolite. Such a behavior has been tentatively ascribed to the body-centered structure of the zeolites whereby compression of the structural framework is allowed without influencing the rigidity of the networks. Based upon the experimental results, the authors postulate that such a behavior will be absent in the case of non-body-centered aluminosilicate zeolites. The manuscript is well-articulated, the discussion and conclusions being well supported by the experimental data. Perhaps it will be useful for the reader to know what is the experimental error in the determination of the unit cell parameters and cell volumes with the pressure. based upon the foregoing, I recommend the publication of this manuscript after the authors address the comments indicated above.
We thank the reviewer for their positive comments and their recommendation. To address their comment regarding the errors in unit cell parameters and cell volumes we have edited the manuscript as follows: In the Materials and Methods section we have included the following sentences: "The error in the calculated unit cell parameters were determined in the TOPAS Academic software. It was seen that the errors were consistently <0.004 Å and had negligible influence on the trends observed. Tables containing the full  Comments to the Author(s) Dear Authors, This a great work in zeolites world. I have a couple thoughts to share. First, do you think the discrepancy of compressibility betweeen ZK-5, RHO, and NaX is simply due to the larger beta-cage, that is 12-ring vs. 8-ring?
We thank the reviewer for their compliments and the issues they raise. The difference in compressibility is due to the different aperture and cavity sizes present in zeolites Na-X, ZK-5 and RHO. Due the larger cavity in zeolite Na-X there is more space available for the 18C6. In order to clarify this point that we were trying to make we have included the following sentence to the Discussion section: "This is believed to be due to the fact that zeolite Na-X has a larger aperture (12-ring) and cavity size compared to zeolites meaning there is more available space for the 18C6 molecule. Furthermore, this suggests that in the assembly of zeolite Na-X the 18C6 express weak interactions with the framework, indicative of a space-filling species." Also, it's well accepted that the zeolite pores do possess certain flexibility, it would be nice to reference some simulation or other experimental data for comparison.
We thank the reviewer for noting this. Indeed, we have included multiple references regarding zeolite framework flexibility, and a mixture of simulated and experimental work. For example : We have compared how extra-framework content influences compression [11, 13, 25-28, 49, 50], how the data compares to what has been seen previously for the same zeolites [15-17, 26, 52], how the flexibility windows of the zeolites compare to another zeolite (EMC-2) made with 18-crown-6 ether [33]. In Table 2 3) In the Discussions section we have included some further discussion comparing the flexibility and phase transitions seen in ANA-type zeolites and zeolites  The nature of all the symmetry transitions are summarised in table 2. The included discussion is as follows: "This is even true when the ambient space group differs between zeolites of the same framework type, as is the case for the ANA-type zeolites [20][21][22][23]. For ANA-type zeolites the lower symmetry form is consistently triclinic, and according to geometric simulations by Wells et al. [24] it is the edge of the flexibility window which controls these pressure-induced symmetry changes. However, herein for the higher symmetry/low pressure forms of zeolites RHO and ZK-5 we see the pressureinduced symmetry change before the window edge is reached."